This invention pertains to surface acoustic wave (SAW) devices and more particularly to an arrangement for improving SAW device corrosion resistance and reliability.
A surface acoustic wave device includes a piezoelectric substrate which has a highly polished surface and metallized SAW transducer patterns on that surface. The transducers are disposed on the polished surface of the piezoelectric substrate such that they are acoustically coupled to one another. The transducer patterns consist of metal strips which couple electrical signals to acoustic signals through the piezoelectric effect, and summing busses which connect the strips to form a filter function and which act as the external electrical connections to the strips. The metal strips of the SAW device, if exposed to the environment directly, can become contaminated and corrode. Passivation of the SAW devices including metal strips has been attempted, but making an electrical connection to these metal strips is a problem since a hole, or via, must be cut through the passivation layer to make electrical connection to the metal strips. Holes in the passivation material allow the environment to attack and contaminate the metal strips. Prior art passivation techniques have relied on holes etched through the passivating layer in order to establish DC connection to the underlying circuitry. Contaminants can adversely affect the performance of these SAW devices by absorbing acoustic energy or by reacting chemically with the transducer metallization.
Therefore, it is an object of the present invention to provide an SAW device which is passivated against chemical attack and having less stringent manufacturing and packaging requirements.